There is an increasing interest in adopting Fresnel lenses which are diffractive lenses for certain ophthalmic applications in lieu of more conventional refractive lenses. Among the reasons for the increased interest in Fresnel lenses for ophthalmic applications is to increase lens optical power and/or reduced lens thickness and therefore they are also referred to as “thin lenses with high power”. One problem which has slowed the industrial development of Fresnel lenses for ophthalmic applications is related to their fabrication. A Fresnel lens conventionally has a so-called structured surface or side including a plurality of concentric ridges of different thickness and dihedral angles which collectively focus the lens and a non-structured surface or side. Fresnel lens like refractive lenses may have a variety of powers. The structured surface of the Fresnel lens can be provided on a planar, convex or concave side thereof.
While it is possible to use an uncoated Fresnel lens as an ophthalmic lens, an uncoated structured side is the source of a host of problems, relating to the structured surface or side of the lens being both unsightly when worn and impractical from the standpoint of user care. To avoid such drawbacks it has been generally agreed that the structured surface or side of the Fresnel lens needs to be coated for reasons of aesthetics and ophthalmic lens care.
The coating of the structured surface or side of a Fresnel lens poses a significant problem in the fabrication of Fresnel lenses for ophthalmic applications. Conventional coating processes widely employed for coating ophthalmic lenses such as spin coating, dip coating or flow coating are inapplicable for coating the structured surface or side of a Fresnel lens because such conventional coating processes cannot be adapted to produce acceptably smooth coated structured surfaces, substantially devoid of waviness.
Various techniques are known for use in making Fresnel lenses which include a layer overlying the structured surface of the Fresnel lens. These techniques include overmolding, casting and BST (back side transfer). Fresnel lens structures with a layer covering the structured surface are disclosed in EP 1 830 204, US2008/00947 and US2004/0263982. None of these disclose a fully satisfactory process for producing coated Fresnel lenses devoid of optical and cosmetic defects, especially when the Fresnel structure height of the lens is in excess of 30 μm.
It has been discovered that it is possible to achieve good quality coating of the structured surface or side of a Fresnel lens from the optical and ‘cosmetic’ standpoints with a so-called “press coating” process such as disclosed in the assignee's published application EP 1 701 838 and counterpart U.S. published patent applications US2005/140033 and US2007/296094, the contents of which are incorporated by reference.
The foresaid patent applications teach the coating of fined (or fine ground) lens blanks to avoid having to polish the lens blank which is a lengthy and costly step in the fabrication of ophthalmic lenses. The unpolished lens blank typically has a roughness (Rq) from 0.01 to 1.5 μm and most commonly about 0.5 μm. A cured coating 1 to 50 μm thick and more commonly less than 5 μm is applied to the unpolished fined lens surface in accordance press coating process disclosed therein.
In the press coating process a requisite amount of a liquid, curable coating composition is deposited on a molding surface of a coating mold part or the unpolished fined surface of the lens blank to be coated. The mold part has a matching curvature to that of the unpolished fined surface of the lens blank. In practice the lens blank is mounted on a balloon, bladder or other inflatable membrane in communication with an air accumulator connected to a source of pressurized air. The pressurized air supplied to the accumulator expands the balloon or bladder to apply the lens blank against the matching surface of the mold surface with a pressure of about 84 kPa (or about 12.2 psi) thereby spreading the curable coating liquid uniformly over the unpolished fined lens blank surface. Thereafter the coating liquid is cured in situ and the pressure is released and the coated lens blank is removed from the mold. The resulting coated lens has very good light transmission and low haze and eliminates visible fining lines when examined with an arc lamp.
Given the topology of the structured surface of a Fresnel lens the press coating process for an unpolished fined lens blank surface is not directly applicable. In fact attempts at applying press coating process to the coating of the structured side of surface of a Fresnel lens revealed two kinds of defects, so-called cosmetic defects and optical defects.
These defects are caused by shrinking of the coating composition applied to the Fresnel lens blank during curing of the coating composition: the greater the Fresnel structure height the greater resulting shrinkage of the coating composition.
It is advantageous to have a thin coating on Fresnel lenses in order to reduce the overall thickness of the resulting lens. But coatings not thick enough to cover satisfactorily the structured surface of the Fresnel lens blank produced surfaces which were not acceptably smooth to provide good optical quality. While good surface quality can be obtained with coatings of the order of 1 to 2 mm such coating thicknesses are detrimental to the desired reduced overall thickness of the lens.
Another problem encountered was the formation of so-called cosmetic ring void defects, which occur in the peripheral region of the coated structured side of the Fresnel lens blank and consist of rings or non-circular irregular contour lines of variable radial distances from the centre of the lens blank such that the rings or contours lines intersect one another at one or more locations.
Copending U.S. patent application Ser. No. 12/651,646 filed Jan. 4, 2010, assigned to the assignee/applicants of the present application, the content of which is incorporated by reference, teaches that it is possible to obtain a good optical surface quality with an acceptably smooth coated structured surface, that is reduced surface roughness, without excessively thick coatings of the order to 1 or 2 mm, by adapting the coating to the height of the structured surface or Fresnel surface of the Fresnel lens and specifically by adopting coating thicknesses which are greater than about 1.5 times the height of structured surface, or the Fresnel structure height, but less than about 5.0 times the height of the structured surface, or Fresnel structure height, of the lens. In practice the resulting surface roughness can be made equal to or even less than 300 nm. This results in coating thicknesses in the range of about 100 to about 600 μm.
Copending U.S. patent application Ser. No. 12/651,646 also teaches the cosmetic ring void defects could be totally eliminated by directing incident radiation, here UV radiation, at the Fresnel lens and not at the glass mold contrary to conventional curing procedure. The application teaches that the coatings in the range of thickness between about 100 and about 600 μm are particularly sensitive to shrinkage.
According to the aforesaid copending application directing the incident UV radiation to the Fresnel lens the shrinkage develops in the direction opposite to that of the incident radiation. Thus with shrinkage developing from the smooth glass mold surface towards the Fresnel structure surface, shrinkage is uniform and produced an acceptably smooth surface. As a result, no irregular shrinkage rings or ring void defects are visible. The copending application also teaches that when the incident UV radiation is directed at the glass mold, the shrinkage of the coating liquid develops from Fresnel structure surface, again in a direction opposite to that of the incident radiation and results in visible irregular shrinkage rings or ring void defects caused by the development of shrinkage from the irregular Fresnel surface of the lens.
According to the invention disclosed in copending application Ser. No. 12/651,646 there is provided a method for coating a Fresnel lens or lens blank, e.g. for use as a ophthalmic lens blank, comprising providing a Fresnel lens blank having a structured surface and a non-structured surface, providing a transparent mold part having molding surface substantially matching the base curvature of the Fresnel lens, depositing a metered quantity of coating resin or coating material between the molding surface and the structured surface of the Fresnel lens, applying pressure between the Fresnel lens blank and the mold part while maintaining the distance between the molding surface and the structure surface so that the thickness of the coating is between about 1.5 and about 5 times the height of the structured surface, or the Fresnel surface height, and about 5 times the height of the structured surface, or Fresnel surface height, and curing the resin coating in situ by directing the incident UV radiation at the Fresnel lens side, not at the glass mold side.
One or more the following features may also be adopted in such a process.
The pressure applied between the mold and the Fresnel lens blank may be between about 2 and about 5 psi (or about 13.8 and about 34.5 kPa).
The thickness of the coating may be between about 1.5 times the Fresnel structure height and about 5 times, or preferably 3 times, the Fresnel structure height.
The coating thickness may be between about 75 and 750 μm.
The Fresnel structure height of the Fresnel lens blank may be between about 20 μm and about 500 μm.
A plurality of circumferentially spacers may be disposed between the mold and the Fresnel lens having an axial length between about 80 and about 800 μm and particularly between about 100 and about 600 μm.
The difference in the refractive index of the Fresnel lens bulk material and the cured coating material may be greater than 0.05, greater than 0.06 and preferably greater than 0.10, or even greater than 0.15.
The refractive index of the cured coating material may be between about 1.45 and about 1.55, and even between 1.38 and 1.55 and the refractive index of the bulk material of the Fresnel lens blank is between about 1.59 and about 1.74.
The coating formulations (or coating materials) may be UV curable compounds selected from the group consisting of UV curable (meth)acrylic compounds, epoxy acrylic compounds, epoxy compounds, polyurethane acrylic compounds, fluoro-acrylic compounds, and any mixture of the aforesaid compounds. The Fresnel lens bulk material may be thermoplastic or thermosetting transparent polymer, and preferably a thermoplastic polycarbonate or a thermosetting polymer formed by curing compounds comprising thiourethane group(s) and/or episulfur group(s).
Typically the hardness of the coating may be between about 60 Shore A. and about 90 Shore A and the surface roughness of the cured coating material may be between about 0.01 μm and about 1.5 μm
The elastic modulus of the cured coating may be greater than 4 mPa.
The difference in the refractive index of the Fresnel lens bulk material and the cured inner coating material may be between about 0.05 and about 0.40, and particularly between about 0.10 and about 0.35, even more particularly between about 0.15 and about 0.30.
The refractive index of the cured inner coating material may be between 1.38 and 1.55, and more particularly between 1.40 and 1.50 to provide a wide-range of optical power, and the refractive index of the bulk material of the Fresnel lens blank may be between 1.59 and 1.74.
The inner coating material may be, for example, UV curable fluorinated (meth)acrylic formulation such as MY-1375.
Bi-focal, tri-focal and other multifocal ophthalmic lenses, including refractive and/or diffractive bi-focal, tri-focal and other multifocal ophthalmic lenses suffer from the drawback of refractive lenses in general, namely that of increased thickness, which in the case of such lenses is typically located at the near and/or middle vision segment(s), compared with the relatively reduced thickness of the (typically) distance vision zone.
One way of reducing the thickness of the near and middle vision segment(s) or insert(s) is use a higher refractive index material for that segment. For that purpose the lens blank has a structured surface or side with a recess to accommodate a segment made of a different (and normally higher index material) and bonding the segment in the recess to the structured surface of the lens blank. Such a process requires a relatively large number of steps and therefore increases manufacturing costs.
Air-encapsulated Fresnel lenses are known and they high optical power owing to the large difference of the refractive index of the bulk material of the lens and that of air. Such air-encapsulated Fresnel lenses have very high haze levels and also require a specific type of frame for mounting the same for use as eyeglasses.
WO2009/079342 inter alia discloses a bi-focal or multifocal ophthalmic lens comprising one for more diffractive visions zones. A resin layer can be cast on top of the structured surface of the preform to form a multifocal lens. The resulting ophthalmic lens has a relatively low optical power.